1. Field of the Invention
The present invention relates to an apparatus for processing a powdered or particulate product being fed onto a substantially horizontally arranged thin, perforated plate which is vibrated while blowing gas upwardly through the perforations or openings thereof. In this manner the powdered or particulate product may be maintained in a fluidized condition while being moved along the plate in a so-called vibrated fluid bed. Thereby it is possible to obtain a good contact between the particles of the product and the processing gas which flows up through the perforations of the bed plate, and which may, for example, be heated air. Such fluidizing apparatus is generally used in connection with drying and/or agglomeration of powdered products, for example for additional drying of spray dried products. It should be understood, however, that in principle the apparatus to be described may also be used in connection with any other processing of a powdered or particulate product with gas flowing upwardly through the perforations.
2. Description of the Prior Art
In order to reduce the amount of the product falling down through the perforations of the vibrating bed plate when moved along that plate in a fluidized condition, these perforations or openings--which are normally formed by punching--must be relatively small. However, a normal condition for forming holes or perforations in a plate by punching is that the diameter of the holes should not be smaller than the thickness of the plate. Consequently, in order to produce a plate having small (for example of the order of 1 mm) holes or perforations punched therein it is necessary to use a relatively thin plate (for example with a thickness of less than 1 mm).
Such a thin plate does not possess a sufficient inherent rigidity, and therefore it is necessary to provide some kind of stiffening. In a known embodiment the bed plate is stretched over a frame-like part and fastened thereto along the edge portions of the plate by welding. Such welding may cause creation of flaws in the plate due to stress concentrations. It is also known to support the plate by ribs positioned at the bottom surface thereof. Such ribs give raise to the formation of undesired "dead zones" on the plate because the ribs cover and close the holes or perforations in the plate at the positions where they engage with the bottom surface of the plate. When a perforated bed plate of the type described is used for processing perishable products within food industry, for example for drying powdered milk products, the plate must be formed so as to be easy to clean and so as to avoid sharp and not easily accessible corners, narrow pockets, and the like which may collect deposits of the product being treated and thereby create favourable conditions for bacterial growth. These requirements may hardly be fulfilled by using a bed plate supported by ribs as described above, and therefore, in practice a plate of that type is not suited for use in sanitary plants.
Furthermore, when a thin plate supported or stiffened at substantially spaced locations in a known manner is vibrated, the plate will also tend to vibrate with its own fundamental frequencies which are difficult to predict and which may cause that the residence time of the product will be different at various parts of the plate in an unpredictable manner.
Apparatuses of the type described above are known, for example from U.S. Pat. Nos. 3,768,174, 3,821,342, and 4,033,555.